Nonaqueous electrolyte secondary batteries such as a lithium ion secondary battery have a high energy density, and are thus in wide use as batteries for devices such as a personal computer, a mobile telephone, and a portable information terminal. Such nonaqueous electrolyte secondary batteries have recently been developed as an on-vehicle battery.
A microporous film that contains a polyolefin as a main component is used as a separator in a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery (Patent Literature 1). Use of such a separator makes it possible to provide a nonaqueous electrolyte secondary battery having both of good safety and a cycle characteristic.